When someone private messages you, and you are tabbed out of FAF, there's a loud and annoying "popping" sound it makes. This is good - it lets you know that someone has sent a message. I have no problem with this, nor the visual notification, where FAF starts flashing.
However, it notifies when someone closes FAF or the chat window. I suggest you add a check to the code so it won't do this when the message "<user> has left the chat" is sent. It's very annoying to hear that popping sound and rapidly check FAF, wondering if it's a time sensitive PM, and finding that someone simply left the conversation.